An easy way I've found to do subtitles, is simply converting the audio track to a wav and importing it into a program such as cooledit. The times match up perfectly with the ones in the subtitle files, unlike say using PowerDVD, where the times are not as accurate.

Then, you'd create a text file with the extention srt and input the subtitle in this format:

1 (the number of the subtitle)
00:00:04,284 --> 00:00:07,583 (point where the line begins ---> ends)
Watch it, bub! (line being spoken)

That being said, I might give it a try when my SFII DVD is finally done.